1. Field of the Invention
The present invention relates to a piezoelectric device in which a piezoelectric thin film is bonded to a support substrate, and a method for producing the same.
2. Description of the Related Art
In recent years, many piezoelectric devices have been developed. In the piezoelectric devices, a piezoelectric thin film is bonded to a support substrate or the like (for example, refer to Japanese Unexamined Patent Application Publication No. 6-326553, Japanese Unexamined Patent Application Publication No. 2004-343359, and Japanese Unexamined Patent Application Publication No. 2005-252550).
Various methods have been proposed and employed as a method of bonding a piezoelectric thin film to a support substrate. For example, in a bonding method referred to as hydrophilic bonding (refer to Japanese Unexamined Patent Application Publication No. 6-326553), first, an inorganic oxide layer is formed on each of the bonding surface of a thin film and the bonding surface of a support substrate, which have been subjected to mirror finishing. Then, a hydroxyl group is formed on the surface of each of the inorganic oxide layers. Then, the surface of the inorganic oxide layer on the thin film side and the surface of the inorganic oxide layer on the support substrate side are overlaid on each other, and thereby, the inorganic oxide layer on the thin film side and the inorganic oxide layer on the support substrate side are bonded together by hydrogen bonding between hydroxyl groups. Then, heat treatment is performed at 200° C. or higher to eliminate H2O from hydrogen-bonded hydroxyl groups, thus markedly increasing the bonding strength between the inorganic oxide layer on the thin film side and the inorganic oxide layer on the support substrate side.
Furthermore, in a bonding method referred to as activation bonding (refer to Japanese Unexamined Patent Application Publication No. 2004-343359 and Japanese Unexamined Patent Application Publication No. 2005-252550), first, the bonding surface on the thin film side and the bonding surface on the support substrate side, which have been subjected to mirror finishing, are each subjected to sputter etching in an inert gas atmosphere or in vacuum. Thereby, contamination is removed from the surfaces, and the surfaces are activated. In that state, the bonding surface on the thin film side and the bonding surface on the support substrate side are overlaid on each other, and thereby, in a non-heated environment, the bonding surface on the thin film side and the bonding surface on the support substrate side are strongly bonded to each other with an amorphous layer interposed therebetween.
In general, when a piezoelectric body has a resistance lower than a certain value, its characteristics are degraded, and therefore, the piezoelectric body has a relatively large resistivity. Consequently, a large amount of pyroelectric charge is generated by application of heat. For example, the volume resistivity of lithium tantalate, lithium niobate, or the like is high at 108 Ω·m or more. Consequently, unless a structure for releasing the pyroelectric charge from a piezoelectric thin film is provided on the piezoelectric thin film, there may be cases where the electric field due to the pyroelectric charge is stronger than the coercive electric field of the piezoelectric body locally in the plane of the piezoelectric thin film. In a piezoelectric body, in order to obtain sufficient piezoelectricity, spontaneous polarization is necessary. In the case where the electric field due to pyroelectric charge is stronger than the coercive electric field of the piezoelectric body locally, polarization inversion may occur in that region, resulting in a degradation of piezoelectricity of the piezoelectric thin film. Furthermore, the pyroelectric charge may cause electrode breakage to occur in functional electrodes.
Accordingly, in the structures of the piezoelectric devices disclosed in the prior art documents described above, since an insulating layer is disposed between the piezoelectric thin film and the support substrate, the pyroelectric charge is not released, and there is a possibility that the problems due to the pyroelectric charge described above will occur.
In order to prevent the occurrence of the problems due to the pyroelectric charge, it is desirable to provide an electrode structure for releasing pyroelectric charge on a piezoelectric thin film. However, in such a case, since the piezoelectric device has an undesirable electrode structure, the types of device in which such a piezoelectric device can be used are limited. For example, in a radio-frequency device, such as a filter or multiplexer, radio-frequency signals leak through the undesirable electrode structure, resulting in degradation in characteristics. Consequently, it is not possible to provide an electrode structure for releasing pyroelectric charge, and it is difficult to prevent the occurrence of problems due to pyroelectric charge.